Process for the preparation of block copolymers

ABSTRACT

A process is disclosed for manufacturing coupled radial block copolymers of the S-B-Li type having improved optical properties, which process utilizes coupling agents having from about 3 to about 7 epoxy groups per mole.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of the U.S. patent application havingSer. No. 05/196,331 filed on Feb. 14, 1994 now abondoned in whichpriority of a previously filed Belgian application, Serial Number09300142, filed in Belgium on Feb. 15, 1993 is claimed.

BACKGROUND OF THE INVENTION

The present invention relates to a new process for the preparation ofblock copolymers in the presence of agents for coupling polymericblocks, and more particularly involves a process for the preparation ofradial block copolymers formed from polyvinylaromatic conjugatedpolydiene blocks which have been formed using agents comprising epoxygroups.

The present invention also relates to radial block copolymers preparedwith new coupling agents and having improved physical properties withrespect to those of block copolymers prepared with conventional couplingagents.

The coupling of polymer chains terminated by a lithium atom (also calledliving based polymers) is well-known in the field, as are the couplingagents used for such purposes. In general, a polymer chain terminated bya lithium atom is reacted with a compound having two or more functionalgroups capable of reacting with the carbon-lithium bond of the polymerchain.

Depending on whether the intention is to form radial polymers ormulti-branched polymers, coupling agents having more than two reactivesites or reaction groups may be used.

European Patent No. 2012 discloses the use of systems of polyfunctionalcoupling agents in which the addition of a polyvinyl monomer which actsas a non-deactivating coupling agent is used in the first instance, anda di- or trifunctional coupling agent, which may or may not bedeactivating, is used in the second instance. This type of system ofcoupling agents leads to numerous branchings which are difficult tocontrol.

U.S. Pat. No. 4,304,886 discloses the use of mixtures of couplingagents, which may be any coupling agents, in order to obtain a polymerhaving a desired total functionality greater than 2 without having toblend several polymers. However, the patent teaches that such systemsusing mixed coupling agents leads to worse mechanical properties.

U.S. Pat. No. 3,880,954 describes the use of alkylpolyalkoxysilane asthe sole coupling agent, having at least two and preferably three alkoxygroups. However, this type, of agent alone does not produce theproperties desired to be obtained, and moreover it leads to theformation of troublesome by-products, such as the correspondingalcohols.

It is well-known that the residue of the coupling agents remains in thecopolymer formed and is thus capable of causing toxic residues or otherunwanted products in the polymers, which products may be troublesome incertain uses and, in particular, in the foodstuff packaging sector. Thisphenomenon is particularly important when one of the coupling agents issilicon tetrachloride (SiCl₄). Indeed, when SiCl₄ is used as couplingagent, lithium chloride (LiCl) is formed as by-product. The presence ofLiCl is not only detrimental to the optical properties of copolymers(causing unwanted opacity), but also induces thermal aging of thecopolymers.

There is thus a need for the development of a process for thepreparation of block copolymers which utilizes coupling agents which donot form toxic or unwanted products, but which are able to achieve thedesired physical properties, and especially transparency. Transparency,or improved optical properties are an issue when the copolymers containless than sixty percent (60%) by weight of the of the mono vinylaromatic component. The conjugated polydiene block component adverselyaffects the optical properties of the final copolymer when present inamounts greater than thirty percent (30%) by weight.

SUMMARY OF THE INVENTION

The present invention discloses a process for the preparation ofvinylaromatic conjugated diene copolymers with coupling agents whichprovide block copolymers having improved physical properties, includingimproved optical properties.

The aim of the present invention is also the preparation ofvinylaromatic conjugated diene block copolymers utilizing couplingagents which form essentially no residual toxicity.

The process of this invention involves the production of blockcopolymers by the coupling of block base copolymers terminated by alithium atom of the typical formula S-B-Li, in which S is amonovinylaromatic hydrocarbon block and B is a conjugated diene block.The monovinylaromatic hydrocarbon block is present in an amount fromabout 20 to about 60 percent by weight. Improved optical properties insuch copolymers are referred to as decolorization of the blockcopolymers. The process is characterized in that it uses at least onecoupling agent comprising from 3 to 7 epoxy groups per molecule, andpreferably from 4 to 6 epoxy groups per molecule.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 represent molecular weight distributions prepared in graphicalform, as determined by gel permeation chromatography.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the process, a block base copolymer is prepared by copolymerizationof a vinylaromatic monomer to give a first block denoted S, in thepresence of an organolithium compound as a catalyst, and an inerthydrocarbon solvent. A conjugated diene monomer is then added to thereaction medium to give a block copolymer of the type S-B-Li, in which Brepresents the conjugated diene block. The process of the presentinvention is applicable to copolymers having a vinyl aromatic componentcontent of from 20% to 60% percent by weight or the final product. Theconjugated diene block content is greater than or equal to 40% byweight.

The catalyst used is generally an alkyllithium, and can be a branchedalkyllithium such as those having secondary alkyl radicals with 3 to 8carbon atoms. However, n-butyllithium is preferably used for reasons ofease of procurement and storage stability.

The solvents used are generally paraffinic, cyloparaffinic and aromatichydrocarbons and their mixtures. Examples are n-pentane, n-hexane,n-heptane, 2,2,4-trimethylpentane, cyclohexane, cyclopentane, benzene,toluene and xylene. Polar solvents, such as cyclic ethers (THF), acyclicethers, or tertiary amines can also be incorporated in order to obtainthe formation of a specific polymeric microstructure, such as, forexample, an increased amount of vinyl units, as well as random S/Bblocks.

It was noted that at this stage of the process, reacting the block basecopolymer which has been terminated by a lithium atom (called the livingbase polymer) with at least one coupling agent comprising from 3 to 7epoxy groups per molecule, and preferably from 4 to 6 epoxy groups permolecule at a rate of 0.1 to 1 phr (per hundred resin or per hundredrubber) and more preferably at a rate of 0.2 to 0.75 phr of the totalpolymers obtained by coupling, improves the physical properties of theblock polymers, in particular the optical properties, while avoidingincreases in residual toxicity. The optical properties, i.e., theclarity of the final product, become important where the vinyl aromaticcontent is lower than 70% by weight and particularly when lower than orequal to 60% by weight. In accordance with the present invention,utilization of the particular coupling agents results in improvedoptical properties, sometimes referred to as decolorization. Bydecolorization, we mean a resultant product having improved opticalproperties.

One embodiment of the present invention provides a process for thedecolorization of grafted block copolymers formed by the coupling ofblock base copolymers terminated by a lithium atom, of the formulaS-B-Li, in which S is a monovinyl aromatic hydrocarbon block and ispresent in amount from about 20 to 60% by weight, and B is a conjugateddiene block. The process is characterized in comprising the use of atleast one coupling agent having from three to seven epoxy groups permolecule. It is preferred that the coupling agent comprises from four tosix epoxy groups per molecule. The coupling agent is used in the amountof 0.1 to 1 phr of the total polymers obtained by coupling. In anotherembodiment, the coupling agent is chosen from the group consisting ofepoxidized vegetable oils, epoxidized polybutadienes, and the epoxidizedtetrallyether pentaerythritol. When epoxidized vegetable oils areutilized, soybean oil and linseed oil are preferred.

Another embodiment of the present invention provides a process for theproduction of copolymers comprising polymerizing a monovinyl aromatichydrocarbon in the presses of a lithium catalyst to form a base polymerterminated by a lithium atom, adding a conjugated diene in the presenceof a solvent, continuing the polymerization process, and adding to thepolymerization process a coupling agent having from 3 to 7 epoxy groupsper molecule. The amount of monovinyl aromatic hydrocarbon is notgreater than 60% by weight of any final product of the process. Thecoupling agent may be added in a stoichiometric amount. In oneembodiment, the polymerization process is continued at a highertemperature than initiated with the monovinyl aromatic compound.

Another embodiment of the present invention provides a process for theproduction of styrene butadiene copolymers, comprising coupling of basecopolymers terminated by a lithium atom of the formula S-B-Li, in whichS is a styrene block and is present in an amount from about 20 to about60% by weight, and B is a conjugated diene block. The process ischaracterized in comprising the use of at least one coupling agenthaving from 3 to 7 epoxy groups per molecule.

Moreover, it has been unexpectedly found that the present invention canbe utilized to provide copolymers having a broader molecular weightdistribution, which in turn improves rheologic properties. FIGS. 1 to 3illustrate clearly this broadening of molecular weights. These figurescome from chromatographic analysis of the block copolymers by gelpermeation. FIGS. 1, 2 and 3 correspond respectively to examples 2, 4and 6 of Table 1.

Among the coupling agents which may be used in the present invention areagents of the epoxidized vegetable oil type, epoxidized polybutadienes,and, epoxidized tetrallylether pentaerythritol. Polyepoxidized vegetableoils such as epoxidized soybean oil or epoxidized linseed oil comprising3 to 7 epoxy groups per mole, preferably 4 to 6 epoxy groups per mole,are the preferable coupling agents.

The amounts of coupling agent to be used can easily be calculated. Thereaction between a coupling agent having a molecular weight M₁ and afunctionality n, and S-B-Li chains of molecular weight M₂, carried outin a molar ratio of 1:n will theoretically give a copolymer of molecularweight M₁ +nM₂, reduced by the molecular weight of the couplingby-products. Deviations in actual results are due essentially to tracesof impurities, or to heat, which can, for example, deactivate the S-B-Lichains (giving copolymers with a molecular weight of about M₁ in thefinal product). The total amount of coupling agents used is preferablyan amount calculated to couple all the S-B-Li chains, but less can beused if preservation of an increased proportion of the S-B copolymers inthe final product is desired. It is also noted that the amount ofcoupling agents may vary with differences in the epoxy groups number.

The vinylaromatic compound which constitutes the block S of the blockcopolymer can be styrene, vinyltoluene, vinylxylene or vinylnaphthalene;while the conjugated dienes are generally chosen from butadiene,isoprene, methylisoprene and their homologues.

The block copolymer formed according to the process of the invention ispresent in a radial or polybranched form.

The molecular weight (weight-average) of the base copolymer can varywithin wide limits and is generally between about 10,000 and 150,000 andpreferably between 15,000 and 100,000, with the polyvinylaromatic blockrepresenting 20 to 60% by weight, preferably 30 to 50%, and mostpreferably about 40%, of the base copolymer.

The process according to the invention is generally practiced bypolymerizing a first block of vinylaromatic monomer, which is usuallystyrene, at a temperature between about 20 and 60° C. for a period of 20minutes to 1 hour in the presence of cyclohexane solvent.

When all the vinylaromatic monomer has been polymerized, a monomer of aconjugated diene, such as 1,3-butadiene, is introduced into thesolution. This monomer reacts entirely at the living ends of thepolymer.

When this stage of the process has been reached, polymeric chains of thetype S-B-Li have been formed. The coupling agents as defined above arethen introduced. The coupling reaction takes 0.1 to 1 hour at atemperature between 10 and 120° C.

During the coupling step lithium alcoholates are formed which aregrafted to the polymeric chains. Although it is not essential, thesealcoholates may be generally neutralized by addition of an acidcompound.

After the coupling step, any non-coupled living polymeric chains presentcan be deactivated by addition of a customary chain terminator, such asan alcohol or a polyalkylphenol.

An antioxidant system suitable for the final use may be then added. Inprinciple, there should be no coupling agent which has not reacted, anyexcess which has not reacted having been converted into non-toxicresidues by steam removal of the solvent.

The following examples are given for better illustration of the processof the present invention.

EXAMPLES

Styrene was first polymerized in the presence of n-butyllithium catalystand cyclohexane as the solvent. The reaction was started at atemperature of 50 to 55° C. and ended at a temperature of about 60 to65° C.

1,3-butadiene in a cyclohexane solvent was then added. Thispolymerization was carried out at a temperature between 60 and 90° C.When this polymerization ended, living chains of the type S-B-Li hadbeen obtained.

Types and quantities of coupling agents that were then added areindicated in Table 1.

The coupling reaction took 1 hour at 50° C.

The characteristics of the radial block copolymers obtained (stillcalled pure polymer) are also indicated in Table 1.

                  TABLE 1                                                         ______________________________________                                                  Coupling                                                                      Agent    Total   Molecular                                                    Concen-  Styrene weight Coupling                                    Examples  tration  (weight (weight                                                                              index                                       Type      (phr) (d)                                                                              %)      average)                                                                             (c)    Mw/Mn                                ______________________________________                                        1. soybean oil (a)                                                                      0.750    40.3    64700  2.5    1.4                                  2. soybean oil (a)                                                                      0.250    39.4    122685 3.9    1.5                                  3. linseed oil (b)                                                                      0.575    39.4    83600  2.4    1.3                                  4. linseed oil (b)                                                                      0.242    39.3    132360 4.8    1.6                                  5. SiCl.sub.4                                                                           0.153    38.8    112400 3.7    1.3                                  6. SiCl.sub.4                                                                           0.150    39.4    117695 3.9    1.3                                  ______________________________________                                         (a) The soybean oil contains 4.4 epoxy groups per molecule                    (b) The linseed oil contains 5.8 epoxy groups per molecule                    (c) final molecular weight/"base polymer" molecular weight                    (d) "phr" means per hundred rubber or per hundred resin as the case may b

It is interesting to note that a coupling index of 2.5 is obtained whensoybean is used at a concentration of 0.75 phr (example 1); suchconcentration is defined as "stoichiometric" between the base polymerand the functional groups represented only by the soybean oil 4.4 epoxygroups. On the other hand, a coupling index of 3.9 is obtained whensoybean is used at a concentration of 0.25 phr (example 2); in thiscase, such concentration is defined as "stoichiometric" between the basepolymer and all the functional groups available in the soybean oil, i.e.the 4.4 epoxy groups plus the ester functions. When considering thetotal amount of reactive functions it is believed that soybean oil isapproximately decafunctional.

A similar observation can be done with linseed oil, which is consideredas 5.8-functional in example 3 and as 11.8-functional in example 4.

After addition of antioxidants, the copolymers coming from examples 2,4and 5 were then subjected to steaming treatment to remove the solvent.

                  TABLE 2                                                         ______________________________________                                        Examples                                                                              Time (days)                                                                              Haze (%) (1)                                                                             Transmittance (%) (1)                           ______________________________________                                        2       0          5.3        87.8                                                    5          5.5        88.2                                                    10         5.7        88.4                                                    15         5.7        88.3                                            4       0          9.8        89.2                                                    5          10.1       89.0                                                    10         10.7       89.2                                                    15         10.3       89.1                                            5       0          59.5       81.5                                                    5          87.1       79.0                                                    10         86.3       78.8                                                    15         85.1       79.6                                            ______________________________________                                         (1) norm ASTM D 1003C1 (Hazemeter type XL211 "hazegard"/Gardner)         

The evolution over the course of time of the optical properties of thecopolymers of examples 2,4 and 5 are indicated in Table 2. The evolutionover time of the thermal stabilies of the copolymers of examples 4 and 5are indicated in Table 3. These copolymers were subjected to thermalaging under air at 80° C. in a blow drying oven.

                  TABLE 3                                                         ______________________________________                                        Examples     Time (days)                                                                             Yellow index (1)                                       ______________________________________                                        4            0         2.9                                                                 1         3.7                                                                 3         4.3                                                                 5         5.9                                                                 7         6                                                                   11        8                                                      5            0         3.7                                                                 1         5.7                                                                 3         16.5                                                                5         25.2                                                                7         26                                                                  11        40.7                                                   ______________________________________                                         (1) norm ASTM D 1925                                                     

What is claimed is:
 1. A polymerization process comprising:polymerizingstyrene monomer in the presence of a lithium catalyst starting at atemperature of 50 to 55 degrees centigrade and ending at a temperatureof 60 to 65 degrees centigrade in the presence of a solvent for asufficient period of time to convert substantially all of the styrene toform styrene polymer chains terminated by a lithium atom; adding aconjugated butadiene in the presence of a solvent; continuing thepolymerization process at a temperature from 60 to 90 degreescentigrade, wherein the polymerization temperature after the butadieneaddition is higher than the polymerization temperature for styrene; andadding a coupling agent having from 3 to 7 epoxy groups per molecule ata temperature not greater than 50 degrees centigrade.
 2. The process ofclaim 1 wherein the styrene is present in an amount from 30 to 50percent by weight.
 3. The process of claim 1 wherein the styrene ispresent in an amount not greater than 40 percent.
 4. The process ofclaim 1 wherein the lithium catalyst is n-butyl lithium.